Protective coatings for conversion material cathodes

ABSTRACT

Battery systems using coated conversion materials as the active material in battery cathodes are provided herein. Protective coatings may be an oxide, phosphate, or fluoride, and may be lithiated. The coating may selectively isolate the conversion material from the electrolyte. Methods for fabricating batteries and battery systems with coated conversion material are also provided herein.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No.14/272,518, filed on May 8, 2014, and titled “PROTECTIVE COATINGS FORCONVERSION MATERIAL CATHODES,” which is a continuation-in-part of U.S.patent application Ser. No. 13/922,214, filed on Jun. 19, 2013, andtitled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICAL CONVERSIONREACTIONS,” which claims benefit of the following U.S. ProvisionalPatent Applications under 35 U.S.C. § 119(e): U.S. Provisional PatentApplication No. 61/674,961, filed Jul. 24, 2012, and titled “NANOSCALELITHIUM COMPOUND AND METAL ELECTRODES,” and U.S. Provisional PatentApplication No. 61/814,821, filed Apr. 23, 2013, and titled“NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICAL CONVERSION REACTIONS.”Each of the prior applications is incorporated by reference herein inits entirety and for all purposes.

BACKGROUND OF THE INVENTION

Many battery types have been developed and used, with their respectiveadvantages and disadvantages. Due to its high charge density, lithiumhas been used in various battery types. In a rechargeable lithium-ionbattery, lithium ions move from the negative electrode to the positiveelectrode during discharge. Unfortunately, conventional battery systemsand their manufacturing and processes result in relatively high-cost,low-energy-density batteries that do not meet market demands for manyapplications.

Another type of battery employs a conversion material, which undergoes aconversion reaction with lithium or another transport ion. Conversionmaterials may have lower cost and higher energy density thanconventional lithium-ion batteries but some aspects of their performancerequire improvement.

SUMMARY

One aspect is an energy storage device including an anode, anelectrolyte, and a cathode, which includes a plurality of coatedelectrochemically active material particles. Each particle has a corewhich includes conversion material and the coating. In many embodiments,the coating selectively isolates the conversion material from theelectrolyte and the capacity of the active material is greater thanabout 300 mAh/g. In some embodiments, the cathode further includes anion conductor and/or an electron conductor. In some embodiments, thecathode further includes a current collector. In some embodiments, theparticles have a median diameter of between about 100 nm and about 1000nm. In certain embodiments, the particles have a median diameter ofbetween about 200 nm and about 300 nm.

In many embodiments, the conversion material includes a sulfide, oxide,halide, phosphide, nitride, chalcogenide, oxysulfide, oxyfluoride,sulfur-fluoride, or sulfur-oxyfluoride. The conversion material mayinclude a lithium compound of these anions (particularly in thedischarged state) and/or a metal compound of these anions (particularlyin the charged state). Examples of suitable metals include iron,manganese, copper, and cobalt. In some embodiments, the conversionmaterial contains a fluoride such as ferric fluoride, or ferrousfluoride, or LiFeF₃, or Li₃FeF₆, in a charged state.

In various embodiments, the conversion material may be characterized asa metal component and a lithium compound component intermixed with themetal component. In some embodiments, the metal component is iron,nickel or copper. In some embodiments, the lithium compound component isa lithium halide, lithium sulfide, lithium sulfur-halide, lithium oxide,lithium nitride, lithium phosphide, or lithium selenide.

In various embodiments, the median thickness of the coating is betweenabout 0.5 nm and about 15 nm, across all the particles. In someembodiments, the coating may include two or more layers, each layerhaving a median thickness between about 0.5 nm and about 15 nm. Asexamples, the coating may include an oxide, a phosphate, or a fluoride.In some embodiments, the coating as formed is partially lithiated. Insome embodiments, the coating as fabricated is no more than 50%lithiated. In some embodiments, the coating is lithiated byintercalation, alloy incorporation, ionic bond formation, or mixtureformation. In some embodiments, the coating includes aluminum oxide(Al_(x)O_(y)) or aluminum phosphate (Al_(x)(PO₄)_(y)) and/or aluminumfluoride (AlF_(x)), which may or may not be lithiated. For example, thecoating may include Al₂O₃, AlPO₄, AlF₃ and/or stoichiometric variants ofany of these. In many embodiments, the coating is deposited using bathcoating, spray coating, or atomic layer deposition at a temperature lessthan about 300° C. In some embodiments, the coating is deposited usingspray coating or precipitation. It should be understood that the coatingmaterials disclosed herein may vary slightly from the recitedstoichiometries.

The coating may have an electronic conductivity of at least about 10⁻⁸S/cm. In some embodiments, the coating has an ionic conductivity ofiron(II) ions of no greater than about 10⁻⁸ S/cm. In some embodiments,the coating has an ionic conductivity of lithium ions of at least about10⁻⁸ S/cm.

The coating may have a diffusion coefficient for lithium ions betweenabout 10⁻¹⁰ and about 10⁻⁵ cm²/s. In some embodiments, the coating has adiffusion coefficient for iron(II) ions between about 10⁻¹⁴ and about10⁻⁹ cm²/s. In many embodiments, the median coating coverage is at leastabout 90% of the surface area of the particles.

Another aspect of this disclosure is an energy storage device includingan anode, an electrolyte, and a cathode, which includes an ionconductor, a current collector, electrochemically active conversionmaterial, and a coating. In many embodiments, the coating selectivelyisolates the electrochemically active material from the electrolyte, andthe capacity of the active conversion material is greater than about 300mAh/g. In some embodiments, the cathode forms a substantially continuoussheet, substantially coextensive with and overlapping the electrolyte.

In many embodiments of this aspect, the conversion material includes asulfide, oxide, halide, phosphide, nitride, chalcogenide, oxysulfide,oxyfluoride, sulfur-fluoride, or sulfur-oxyfluoride. The conversionmaterial may include a lithium compound of these anions (particularly inthe discharged state) and/or a metal compound of these anions(particularly in the charged state). Examples of suitable metals includeiron, nickel, manganese, copper, and cobalt. In some embodiments, theconversion material contains a fluoride such as nickel fluoride, copperfluoride, ferric fluoride or ferrous fluoride.

In various embodiments of this aspect, the conversion material may becharacterized as a metal component and a lithium compound componentintermixed with the metal component. In many embodiments, the metalcomponent is iron, nickel, copper, or an alloy of one of those metals.In various embodiments, the lithium compound component is a lithiumhalide, lithium sulfide, lithium sulfur-halide, lithium oxide, lithiumnitride, lithium phosphide, or lithium selenide.

The coating may include an oxide, a phosphate, or a fluoride. In someembodiments, the coating is lithiated. In some embodiments, the coatingis partially lithiated. In some embodiments, the coating is at leastabout 50% lithiated. In various embodiments, the coating includesaluminum oxide (Al_(x)O_(y)), or aluminum phosphate (Al_(x)(PO₄)_(y)),and/or aluminum fluoride (AlF_(x)), which may or may not be lithiated.For example, the coating may include Al₂O₃, AlPO₄, and/or AlF₃. In manyembodiments, the coating is deposited using bath coating, spray coating,or atomic layer deposition at a temperature less than about 300° C. Insome embodiments, the coating is deposited using spray coating orprecipitation.

The coating in this aspect may have an electronic conductivity of atleast about 10⁻⁸ S/cm. In some embodiments, the coating has an ionicconductivity of iron(II) ions of no greater than about 10⁻⁸ S/cm. Insome embodiments, the coating has an ionic conductivity of lithium ionsof at least about 10⁻⁸ S/cm. The coating may have a diffusioncoefficient for lithium ions between about 10⁻¹⁰ and about 10⁻⁵ cm²/s.In some embodiments, the coating has a diffusion coefficient foriron(II) ions between about 10⁻¹⁴ and about 10⁻⁹ cm²/s. In someembodiments, the thickness of the coating is between about 0.5 nm andabout 15 nm. In some embodiments, the coating includes two or morelayers, each layer having a thickness between about 0.5 nm and about 15nm.

Another aspect of this disclosure is a method of preparing anelectrochemically active material component of a cathode in an energystorage device by providing electrochemically active material particles,each particle including a core which includes conversion material; anddepositing a coating on each particle, such that the coating includesone of Al_(x)O_(y), AlF_(x), or Al_(x)(PO₄)_(y). For example, thecoating may include Al₂O₃, AlPO₄, and/or AlF₃. In some embodiments, thecoating is deposited by spray coating, or bath coating, or ALD. In manyembodiments, the coating is deposited to a thickness of between about0.5 nm and about 15 nm. In some embodiments, the coating includes two ormore layers, each layer having a thickness between about 0.5 nm andabout 15 nm.

Another aspect is a method of preparing an electrochemically activematerial component of a cathode in an energy storage device by providingelectrochemically active material particles, each particle including acore including conversion material; and depositing a coating on eachparticle, such that the coating is partially lithiated. In manyembodiments, the coating is deposited to a thickness of between about0.5 nm and about 15 nm. In some embodiments, the coating includes two ormore layers, each layer having a thickness between about 0.5 nm andabout 15 nm. In some embodiments, the coating is deposited using spraycoating, bath coating, or ALD. In some embodiments, the coating includesone of Al_(x)O_(y), AlF_(x), or Al_(x)(PO₄)_(y). For example, thecoating may include Al₂O₃, AlPO₄, and/or AlF₃. In some embodiments, thecoating is deposited at a temperature less than about 300° C.

Another aspect is a method of fabricating a cathode of an energy storagedevice, by providing an electrochemically active material layerincluding conversion material on a substrate, and depositing a coatingon the substrate by physical vapor deposition (PVD), where the coatingincludes one of Al_(x)O_(y), AlF_(x), or Al_(x)(PO₄)_(y). For example,the coating may include Al₂O₃, AlPO₄, and/or AlF₃. In many embodiments,the coating is deposited to a thickness of between about 0.5 nm andabout 15 nm. In some embodiments, the coating includes two or morelayers, each layer having a thickness between about 0.5 nm and about 15nm. In some embodiments, the coating is deposited using spray coating,bath coating, or ALD. In some embodiments, the coating is deposited at atemperature less than about 300° C.

These and other aspects are described further below with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic illustrations of electrochemical cells.

FIG. 2A depicts four examples of conversion materials having variousnanodomain and particle formats.

FIG. 2B depicts additional examples of particle and nanodomainstructures that may be employed in ferric fluoride and relatedconversion materials.

FIGS. 3A and 3B are schematic illustrations of a metal migrationphenomenon.

FIGS. 4A and 4B are schematic illustrations of cathodes in accordancewith disclosed embodiments.

FIGS. 5A and 5B are schematic illustrations of an electrolyte reactionphenomenon.

FIGS. 6A and 6B are schematic illustrations of cathodes in accordancewith disclosed embodiments.

FIGS. 7A and 7B are plots of energy density and energy retention forexperimental results in accordance with disclosed embodiments.

DETAILED DESCRIPTION Introduction

The following description is presented to enable one of ordinary skillin the art to make and use the disclosed embodiments and to incorporateit in the context of particular applications. Various modifications, aswell as a variety of uses in different applications will be readilyapparent to those skilled in the art, and the general principles definedherein may be applied to a wide range of embodiments. Thus, thedisclosed embodiments are not intended to be limited to the embodimentspresented, but are to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the disclosedembodiments. However, it will be apparent to one skilled in the art thatthe disclosed embodiments may be practiced without necessarily beinglimited to these specific details. In other instances, well-knownstructures and devices are shown in block diagram form, rather than indetail, in order to avoid obscuring the disclosed embodiments.

All the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings) may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is only one example of a genericseries of equivalent or similar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. § 112(f). In particular, the use of“step of” or “act of” in the claims herein is not intended to invoke theprovisions of 35 U.S.C. § 112(f).

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

Batteries and their electrodes undergo electrochemical transitionsduring discharge and, in the case of secondary or rechargeablebatteries, charge. Due to its high charge density, lithium has been usedin various battery types. A lithium-ion battery may include a cathode ofinsertion material or intercalation material. Insertion or intercalationmaterial may be molecules which may be included between other molecules.In a rechargeable lithium-ion battery, lithium ions move from the anode(or negative electrode) to the cathode (or positive electrode) duringdischarge. Unfortunately, conventional lithium-ion batteries typicallyhave a relatively high cost and low energy density, and therefore maynot meet demands for many applications. The present disclosure concernscathodes having coatings, which may outperform not only intercalationmaterials but also uncoated conversion materials. In some cases, thecoating prevents a metal material from leaving cathode particles and/orprevents electrolyte from detrimentally reacting with components in thecathode particles. As a consequence, the material exhibits increasedcapacity and increased ion conductivity. In one example, the conversionmaterial is ferric fluoride in a lithium-FeF₃ cell and the coating is athin material such as aluminum oxide or lithium oxide.

Cathodes contain materials that participate in reactions that facilitatecharge and discharge to produce electrical energy, and such materialsmay be broadly termed “active materials” or “electrochemically activematerials.” In various embodiments, about 90% of a cathode may includeactive material. Cathodes may contain high capacity active material thatreversibly undergoes a redox reaction at a high rate over many cycles ofcharge and discharge. Such materials are sometimes referred to herein as“conversion” materials. They generally have lower cost and higher energydensity than cathode materials in conventional lithium-ion batteries.Another example of an active material is an intercalation material. Asused herein, a “conversion” material is one that, in an electrochemicalreaction with lithium, may undergo a phase change and at least a portionof the metal component undergoes a change of oxidation state. Aconversion reaction may be represented as Li+AB

LiA+B. A conversion material as used herein is distinguished from aformation material (Li+A

LiA) or an intercalation material (Li+AB

LiAB).

In general, intercalation and/or conversion materials may be used inbattery systems. For example, a cathode material may be used forintercalation or conversion with lithium. Intercalation materials, whichcan be prepared at a macro scale or at a nano scale, typically haverelatively low energy density (e.g., less than about 800 Wh/kg of activematerial).

Conversion materials, in contrast, can provide much higher energydensity (e.g., about 1000 Wh/kg to 2500 Wh/kg of active material).Battery systems and structures utilizing conversion material aredescribed in U.S. patent application Ser. No. 14/207,493, filed on Mar.12, 2014, titled “IRON, FLUORINE, SULFUR COMPOUNDS FOR BATTERY CELLCATHODES”; U.S. Provisional Patent Application No. 61/674,961, filed onJul. 24, 2012, and titled “NANOSCALE LITHIUM COMPOUND AND METALELECTRODES”; and U.S. patent application Ser. No. 13/922,214, filed onJun. 19, 2013, and titled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICALCONVERSION REACTIONS,” all of which are incorporated by reference hereinin their entireties.

Cell Structure Types

In various embodiments, the cathode includes particles ofelectrochemically active material. FIG. 1A shows an example of a cellformat, which includes negative current collector 100, electrolyte 102,cathode 104 a including particles, and positive current collector 106.The negative current collector 100 contacts the electrolyte 102, whichin turn contacts the cathode layer including its constituent particles104 a. The cathode layer of particles 104 a also contacts the positivecurrent collector 106. The cathode may include an additive to improveelectronic conductivity between the active cathode particles and thepositive current collector. Such an additive may be a carbon particle ora mixed electron-ion conductor (MEIC).

An electrolyte separates the anode from the cathode, is ionicallyconductive, and is electronically resistive. It allows transport of ionsthat participate in electrochemical reactions at the anode and cathodeduring charge and discharge. Some batteries contain a singleelectrolyte, which contacts the anode and the cathode. Other batteriescontain two or more physically separate electrolytes having differentcompositions. For example, a battery may have an anolyte contacting theanode and a catholyte contacting the cathode, where the anolyte andcatholyte are separated by in ionically conductive barrier that preventsmixing. Electrolytes typically exist in a liquid, solid, or gel state.Batteries containing two different electrolytes may employ differentphases. For example, a catholyte may be solid while the remainder of theelectrolyte (which may be an anolyte) may be a liquid. As used herein,the term ‘electrolyte’ includes anolyte, catholyte, separatorelectrolyte, and any other material having the above characteristics ina battery.

In some embodiments, the cell includes a single liquid phaseelectrolyte, often in conjunction with an intercalation type anode. Insuch embodiments, a porous separator may be used to prevent contactbetween the anode and cathode.

In some embodiments, a two-phase electrolyte may be used. In oneexample, the cathode includes a liquid catholyte surrounding theparticles of active cathode material, and the catholyte may be separatedfrom the anode by a layer of solid state electrolyte. The catholyte andcathode particles together form a cathode layer as depicted in layer 104a and the electrolyte layer may correspond to layer 102 as shown in FIG.1A. The liquid phase catholyte material is conductive to ions but may beinsufficiently conductive to electrons, in which case an additive, suchas carbon or another electronically conductive material, may be added tothe cathode. In embodiments employing lithium or another metal anode,the solid state portion of the separator or electrolyte may help preventmetal dendrites from forming.

Catholyte compositions may include carbonate electrolytes (e.g., EC(ethylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate),EMC (ethyl methyl carbonate), VC (vinylene carbonate), FEC(fluoroethylene carbonate), PC (propylene carbonate) with salts such asLiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiPF6 (lithiumhexafluorophosphate), LiBOB (lithium bis(oxalato)borate), LiODFB(lithium oxalyldifluoroborate), LiFAP (lithium fluoroalkylphosphate),LiFSI (lithium bis(fluorosulfonyl)imide), etc.) or non-carbonateelectrolytes (e.g., ionic liquids, siloxanes, ethers, nitriles, glymes,etc.). Catholytes may have high voltage and low voltage stability (downto about 1V since the cathode may operate down to a low voltage limit,and up to about 4.5V or greater). Some species for catholytecompositions have high temperature stability, such as siloxanes andother organosilicons.

As mentioned, a solid phase electrolyte may be used. Various solid phaseelectrolytes are described in U.S. Provisional Patent Application No.61/778,455, filed on Mar. 13, 2013, and U.S. patent application Ser. No.14/207,493, filed on Mar. 12, 2014, titled “IRON, FLUORINE, SULFURCOMPOUNDS FOR BATTERY CELL CATHODES” which are incorporated herein byreference in their entireties. Examples of solid phase electrolytes thatconduct Li⁺ include Li₁₀XP₂S₁₂ (LXPS, X=Si, Ge, Sn, and combinationsthereof), Li₁₀SiP₂S₁₂ (LSPS), and LiPON (lithium phosphorus oxynitride).Carbon or carbon materials may be added to improve the electronicconductivity.

FIG. 1B schematically depicts a thin-film cell format. In variousembodiments, a thin-film of electrochemically active cathode material104 b is provided between a positive current collector 106 and a thinprotective layer (not shown but described in more detail below). Thecathode thin film may be a continuous and non-particulate layer. Theprotective layer may contact an electrolyte 102, which may be in solidphase or liquid phase. The electrolyte 102, in turn, contacts an anodeor negative current collector 100. If the electrolyte 102 contacts thenegative current collector 100, it may do so only in the dischargedstate. In a charged state, a metallic anode (not shown) may be depositedin between the electrolyte 102 and the negative current collector 100.Lithium is one example of such a metallic anode material. In variousembodiments, the cathode layer may have a thickness of about 1micrometer or less, or about 500 nanometers or less, or about 200nanometers or less.

Conversion Materials

When considered across a range of states of charge, the conversionmaterial may be viewed as including an oxidizing species, a reducingcation species, and a metal species. These species are sometimesreferred to herein as constituents or components. As disclosed herein,such materials also include a protective coating layer on the surface ofthe cathode that partially isolates or protects the conversion materialfrom the electrolyte and thereby improves conductivity. This sectiondescribes conversion materials in general terms usually withoutreference to coatings, which are described in a later section.

The oxidizing species is typically a strongly electronegative element,compound, or anion. Examples of oxidizing species anions include halideions (fluorides, chlorides, bromides, and iodides), oxide ions, sulfideions, and the like. The reducing cation species is typically anelectropositive element or cation such as lithium, sodium, potassium, ormagnesium, and ions thereof. The metal species is typically lesselectropositive than the reducing cation species. Transition metals aresometimes used as the metal species. Examples include cobalt, copper,nickel, manganese, and iron.

The conversion material may contain two or more oxidizing species, twoor more reducing cation species, and/or two or more metal species. Insome embodiments, the discharged conversion material includes a metalcomponent and a lithium compound component.

Cathode conversion materials may exist in a discharged state, a chargedstate, or an intermediate charge state. In some cases, a battery isdesigned or operated so that full discharge is never attained. Thus, ifthe fully charged conversion material is ferric fluoride (FeF₃), forexample, the “fully” discharged cathode may contain a mixture ofelemental iron (Fe), lithium fluoride (LiF), possibly some ferricfluoride (FeF₃), and possibly some ferrous fluoride (FeF₂). The use of“discharged” or “discharged state” herein is a relative term, referringonly to a state of a conversion material that is more discharged than acharged state of the conversion material. The use of “charged” or“charged state” herein refers to a state of a conversion material thatis more charged than a corresponding discharged state of the material.

In the discharged state, the metal species is generally more reducedthan in the charged state. For example, the metal species may be anelemental state or have a lower oxidation state (e.g., +2 rather than+3). Further, during discharge, the oxidizing species may pair with thereducing cation species and unpair from the metal species. Also duringdischarge, the reducing cation species may tend to move into the cathodewhere it becomes oxidized by pairing with the oxidizing species. Pairingis typically manifest by formation of a chemical bond, such as acovalent or ionic bond.

In certain implementations, the conversion material in the dischargedstate includes an elemental metal material, one or more oxidizingspecies, and a reducing cation material. As an example, the dischargestate may include at least an elemental metal such as iron (Fe) and areducing cation halide such as lithium fluoride (LiF). The components ofthe discharged conversion material may be intimately distributed withone other in the discharged material. These materials may be intermixedor distributed at a scale of about 20 nm or smaller.

In the charged state, the metal species may tend to pair with theoxidizing species, often forming a compound. During charging, theoxidizing species tends to unpair from the reducing cation species andpair with the metal species. The reducing cation species tend to moveout of the cathode and migrate and/or diffuse to the anode, where theyexist in a more strongly reduced state (e.g., as an elemental metal suchas lithium metal, or lithium inserted in a matrix of carbon or silicon).

As an example, during charge, elemental iron (Fe) may pair with fluorineanions (F⁻) to form ferric fluoride (FeF₃) and/or ferrous fluoride(FeF₂). The cathode may also include LiFeF₃, Li₃FeF₆, or similarlithiated compound in a charged state. In some examples, the lithiatediron compound has a lithium to iron atomic ratio of between about 1:1and about 3:1. Concurrently, fluoride anions (F⁻) may unpair from areducing cation metal species such as lithium of lithium fluoride (LiF).The newly freed lithium cation (Li⁺) may then migrate and/or diffuse tothe anode, where it is reduced to elemental lithium (Li) or a lithiumintercalation material.

Chemical Reaction and Properties

In the charged state, the conversion material contains a compound of ametal. In some embodiments, the electrochemical charge-dischargereaction at the cathode may be represented, without stoichiometryconsiderations, by the following reaction:M+LiX

MX+Li⁺ +e ⁻  (1)where M is the metal species and X is the oxidizing species, e.g., ananion or electron-rich species of an element such as a halide, oxygen,sulfur, phosphorus, nitrogen, selenium, or a combination of suchelements. In a specific example, the oxidizing species is a combinationof a halogen ion and a chalcogen ion (e.g., fluoride and sulfide). Incertain variations of the above chemical reaction, lithium is replacedwith sodium (Na⁺), potassium (K⁺), magnesium (Mg⁺), or anotherelectropositive metal ion. Certain disclosed embodiments use a redoxreaction of lithium ions with a metal fluoride as a source of energy incathode materials.

The metal compound MX present in the charged cathode material may reactwith lithium ions according to a discharge path described above.Typically, the discharge reaction is associated with a relatively largeGibbs free energy when considering the full cell reaction Li+MX→LiX+M.The Gibbs free energy corresponding to the cell voltage of the reactionis given byΔG _(r×n) =−E*n*F  (2)where E is the voltage, n is the number of electrons that react, and Fis the Faraday constant. In certain embodiments, the Gibbs free energyof the reaction is at least about 500 kJ/mol or at least about 750kJ/mol, or at least about 1 MJ/mol. This provides a very high availableenergy for a battery and compares favorably with that of a standardlithium insertion (or lithium intercalation, depending on the electrodematrix) cathode material, such as lithium cobalt oxide (LiCoO₂), lithiummanganese oxide (LiMnO₂), lithium titanate (Li₂TiO₃), and the like usedin conventional lithium ion batteries.

In certain implementations, the voltage of a fully charged cathode asdescribed in the disclosed embodiments is at least about 2.0V comparedto a lithium metal electrode, or at least about 3.0V compared to alithium metal electrode, or at least about 4.0V compared to a lithiummetal electrode, or at least about 4.5V compared to a lithium metalelectrode.

Electrode Capacity

The conversion materials disclosed herein combine during discharge withmultiple lithium ions per transition metal. During charge, intercalationreactions involve at most one lithium ion per transition metal (e.g., aslithium is reduced from Li⁺ to Li⁰, cobalt oxidizes from Co³⁺ to Co⁴⁺),whereas in conversion materials such as those with ferric fluoride(FeF₃), three lithium ions react per transition metal. In fact, mostinsertion compounds react half a lithium ion per transition metalbecause the electrode structure becomes unstable if more than ½ of thelithium is extracted. Thus, the transition metal electrode materialsdisclosed herein provide a significantly higher capacity (e.g., about700 mAh/g or greater) than conventional electrode materials (e.g., 140mAh/g for LiCoO₂). The capacity is available even at high rates and overmany cycles when the electrode possesses suitably high ionic andelectronic conductivity as disclosed herein.

In certain embodiments, the cathode conversion material, as fabricated,has a specific capacity of at least about 600 mAh/g of the fully chargedcathode material. In some embodiments, the cathode material maintainsthis fully charged capacity over multiple cycles. The fully chargedmaterial is the stoichiometric metal compound, MX. For example, thecathode material may maintain a capacity of at least about 600 mAh/gwhen discharged at a rate of at least about 200 mA/g of fully chargedcathode material.

In some implementations, the material maintains this capacity at higherdischarge rates of at least about 600 mA/g of fully charged cathodematerial. In certain embodiments, the material maintains the capacity atdischarge rates of up to about 6000 mA/g of fully charged cathodematerial. This discharge rate may be maintained at a constant value ormay vary over discharge without dropping below, e.g., 200 mA/g.

In some embodiments, the cathode material maintains a high capacity athigh rates (e.g., at least about 600 mAh/g at 200 mA/g) over multiplecharge-discharge cycles. High capacity performance may be achieved whencycling over a range of temperatures, e.g., from about 0° C. to 100° C.,or about 20° C. to 100° C. In some cases, the electrode material is ableto maintain such a high rate capacity over about 10 cycles or more.Often it will be able to maintain this high rate capacity even longer,e.g., over about 20 cycles or more, or over about 50 cycles or more, orover about 100 cycles or more, or over about 500 cycles or more. In eachcycle, the cathode material discharges the full 600 mAh/g charge. Suchcycling may be conducted such that the voltage of the cathode is between4V and 1V vs. Li/Li′. In some embodiments, the charge rate may be higherthan 200 mA/g, higher than 600 mA/g, or higher than 6000 mA/h, and thematerial maintains a capacity of about at least 600 mAh/g. High capacityperformance may be achieved when cycling over a range of temperatures,e.g., from about 0° C. to 100° C., or about 20° C. to 100° C.

In certain embodiments, the conversion material provides a capacity ofgreater than about 350 mAh/g of active material when cycled between 1Vand 4V vs. a lithium metal anode at about 100° C. with acharge/discharge rate of 200 mA/g. In other embodiments, the electrodematerial provides a capacity of greater than about 500 mAh/g, or greaterthan about 600 mAh/g, or greater than about 700 mAh/g. In each case, thecapacity value is for the active material cycled in the voltage range of1V to 4V vs. a lithium metal anode when cycled at about 100° C. with acharge/discharge rate of 200 mA/g. In another embodiment, the electrodematerials described herein provide a capacity of between about 350 mAh/gand about 750 mAh/g when cycled between 1V and 4V against a lithiummetal anode at about 100° C. with a charge/discharge rate of 200 mA/g.In another embodiment, the electrode materials may have a specificcapacity of greater than about 400 mAh/g when discharged between 1V and4.5V vs. a standard lithium metal electrode (Li/Li+) at a rate of 400mA/g and a temperature of 120° C., or between 1.5V and 4V vs. Li at arate greater than 1 C and a temperature above 50° C.

In certain implementations, a battery containing a cathode as describedherein has an energy density of at least about 50 Wh/kg or between about50 and 1000 Wh/kg when measured at a temperature of 100 degrees Celsiuswhen cycled between 1V and 4V vs. Li and at a current density of atleast about 200 mA/g of cathode active material. In another embodiment,a device as described herein has an energy density of between about 100and 750 Wh/kg when measured as described. In another embodiment, adevice as described herein has an energy density of between about 250and 650 Wh/kg when measured as described. In another embodiment, adevice as described herein has an energy density of greater than about250 Wh/kg when measured as described. As used herein, energy density isthe energy density at the device level; i.e., the total energy stored inthe device divided by the mass of the device, where the mass of thedevice includes the mass of the anode, cathode, electrolyte, currentcollectors and packaging of the device. From a volumetric perspective,in certain embodiments, the device has an energy density of at leastabout 600 Wh/L under the conditions set forth above. While many of theparameters presented herein are presented for electrodes operated at atemperature of 100 C, it should be understood that the parameters may insome instances be achieved at lower temperatures, such as 60 C, 30 C, 10C, or 0 C.

In certain embodiments, a cathode as described herein has an electrodeenergy density of between about 500 and 2500 Wh/kg when measured at atemperature of 100° C. In another embodiment, a cathode as describedherein has an electrode energy density of between about 800 and 1750Wh/kg. In another embodiment, a cathode as described herein has anenergy density of between about 1000 and 1600 Wh/kg. In anotherembodiment, a cathode as described herein has an energy density ofgreater than about 1000 Wh/kg. As used herein, electrode energy densityis the energy density at the electrode level; i.e., the total energystored in the device divided by the mass of the cathode in thedischarged state, where the mass of the electrode includes the mass ofthe electrochemically active conversion material, lithium, positivecurrent collector, and any electrochemically inactive components in thecathode such as ion or electron conductor additives and any protectivecoating on the active material.

In some cases, cathodes fabricated from high capacity conversionmaterials have an average discharge voltage greater than about 2V vs.lithium metal when discharged under above conditions (100° C. and acharge/discharge rate of 200 mA/g). In some embodiments, cathodesfabricated from such conversion materials have an average dischargevoltage greater than about 2.2 V vs. lithium metal when discharged underabove conditions.

Voltage hysteresis is the difference between the discharge voltage andthe charge voltage, both varied as a function of state of charge. Itrepresents the inefficiency of the battery—energy lost to heat, oftendue to sluggishness of either ion transport or reactions. Theinefficiencies are manifest as overvoltages required to drive thereactions, which cause the discharge voltage to be lower than the opencircuit voltage and the charge voltage to be higher than the opencircuit voltage. Low hysteresis means that the battery is efficient.

In certain embodiments, devices employing the conversion cathodematerials described herein provide an average voltage hysteresis of lessthan 1V in the voltage range of 1V to 4V vs. a lithium metal electrodeat about 100° C. with a charge/discharge rate of 200 mA/g. In anotherversion, such devices provide an average voltage hysteresis of less than0.7V when cycled between 1V and 4V vs. a lithium metal electrode atabout 100° C. with a charge/discharge rate of 200 mA/g. In anembodiment, the devices provide an average voltage hysteresis of lessthan about 1V when cycled between 1V and 4V vs. a lithium metalelectrode at about 100° C. with a charge/discharge rate of 600 mA/g. Inan embodiment, the devices provide an average voltage hysteresis of lessthan about 1V when cycled between 1.5V and 4V vs. a lithium metalelectrode at about 50° C. with a charge/discharge rate of 200 mA/g. Incertain embodiments, this hysteresis level is maintained for at least 10cycles, or at least 30 cycles, or at least 50 cycles, or at least 100cycles.

Function and Structure of Conversion Materials

The cathode material that contains an elemental metal or alloy and alithium compound (in a discharged state) or a metal compound (in thecharged state) may be provided in the form of extremely small particlesor nanodomains.

The size scale of the conversion material components in a charged ordischarged state may influence the relevant electrochemical propertiesof the materials. Conversion materials with components separated by verysmall distances, sometimes on the order of the atomic scale, may possesscertain performance benefits as compared to conversion materials withcomponents separated by greater distances. In some embodiments, thecomponents are separated by a distance no greater than about 20 nm. Theterm “nanostructured” is sometimes used to refer to conversion materialsin charged or discharged states in which the component materials areseparated from another at a scale of about 20 nm or less.

In some embodiments, in the discharged state, the conversion materialcontains discrete domains of an elemental metal (or an alloy thereof)and a lithium compound. In various embodiments, these domains arecontained in particles, which may be coated as described herein. Thedomains may be in the interiors or cores of the particles.

Various components of the conversion material may be mixed and/orotherwise exist at nanostructured scale. The individual domains may benanodomains. Nanodomains may have an average or median characteristicdimension of about 20 nm or less, or about 10 nm or less, or about 5 nmor less. Using ferric fluoride (FeF₃) as an example conversion material,the nanodomains may be primarily iron (Fe) and lithium fluoride (LiF) inthe discharged state. In the charged state, the nanodomains areprimarily ferric fluoride (FeF₃). Domains may be compositionallyhomogenous (e.g., containing exclusively metal species) or inhomogeneous(e.g., composed of a combination of metal species, oxidizing species,and reducing cation species). In both states, the nanodomains may becrystalline or amorphous/glassy.

In some embodiments, the discrete domains of metal or alloy are presentin small particles or other discrete structures. In some embodiments,the discrete domains of metal or alloy are embedded in a continuousmatrix of the lithium compound. In some embodiments, the domains of thecathode have a very tight distribution, e.g., a standard deviation ofabout 50% or less. In some implementations, at least about 90% of thedomains in the electrode have a characteristic dimension of betweenabout 1 nm and about 5 nm. In some embodiments, the domains'characteristic dimension has a d₅₀ value of about 20 nm or less, orabout 10 nm or less, or about 5 nm or less, where d₅₀ is defined as thecharacteristic dimension at which 50% of the domains are smaller. Thedomains may be present in these sizes at any point in the life of thecathode. In some examples, the domains are present in these sizes in thecathode as fabricated. In some examples, the domains are present inthese sizes after the first discharge of the cathode, or after the firstfull charge/discharge cycle of the cathode. In certain embodiments, theaverage size of the domains of the cathode do not vary in characteristicdimension by more than about 500%, or by about 100% over multiple cycles(e.g., 10 cycles, 50 cycles, 100 cycles, or 500 cycles).

In the charged state, the cathode conversion material may maintain thegeneral morphological characteristics present in the discharged state.These characteristics include component separation distance (e.g.,particle or crystallite size), matrix structure (e.g., glassy oramorphous), etc. In certain embodiments, a conversion material has aglassy or amorphous morphology, which is associated with high cationicand/or electronic conductivity. In some cases, the material will expandin the discharged state. Depending on the material, the volume changemay be about 5% or greater, or about 10% or greater.

In various embodiments, the conversion material is formed or mixed suchthat its components are separated on a scale of about 1 nm or less, andsuch materials may be characterized as glassy or amorphous. A glassymaterial may be viewed as one that is substantially non-crystalline,substantially uniform in composition, and substantially lacking inlong-range order. In some examples, a glassy conversion material issubstantially homogeneous (compositionally and/or morphologically)within a volume of 1000 nm³.

In one example, ferric fluoride (FeF₃) in a charged conversion materialmay be characterized by a glassy or amorphous structure and beingsubstantially homogeneous with no or minimal crystalline structure. Insome examples, in the discharged state, the conversion material mayinclude a glassy compound of lithium, sodium, and/or magnesium. Suchglassy or amorphous structures may be provided as particles, layers,etc. Within the particles or layers, the domains of component metal,oxidizing, and reducing cation species are, on average, separated fromone another by a distance no greater than about 20 nm. In some cases,particles having a glassy or amorphous state may be substantiallyunagglomerated. In other cases, at least some of the particles formagglomerates.

The extremely small constituent separation distances described hereinprovide a relatively short diffusion/migration path for the lithium orother electropositive ions to move from the outside of a particle ordomain to the reactive metal compound sites within a particle or domainduring discharge, or from a lithium compound within the particle ordomain to the particle or domain surface during charge. During charge,lithium ions may leave lithium fluoride, for example, and transport tothe exterior of the domain where they contact the electrolyte. Afterleaving a particle or domain, a lithium ion may have to contact someother ion conductive matrix in the electrode before reaching theelectrolyte.

Conversely, on discharge, lithium ions undergo a journey from theelectrolyte of the body of the electrode, where they may travel somedistance before reaching a destination domain, which they enter and passinto before finding a reactive metal compound site. Only after thismultistage transport does the lithium ion participate in the redoxreaction to generate electrochemical energy (discharge). The reversepath is traversed during charge. Using small separation distances ofactive material permits the cathode to operate with improved rateperformance that is not available in conventional batteries.

A further benefit derived from the extremely small compound separationdistances is the comparatively shorter diffusion distance between themetal atoms and the anions. As the metal and anion atoms are larger andmore massive, their transport is generally slower than that of lithium.The provided nanostructure puts metal atoms in close proximity toanions, reducing the distance they must diffuse.

An additional challenge to realizing the potential benefits ofconversion materials arises from the high surface area to mass ratio ofvery small particles. The large surface area, as a function of mass ofreactive material, results in a relatively large fraction of the activematerial converting to a solid electrolyte interface (SEI) layer, whichextracts much of the available lithium and presents it in an unusableform. It also therefore results in a relatively short cycle life as theSEI layer may continue to grow for a few cycles. The SEI that formsaround a particle which undergoes significant volume changes duringcycling may sometimes crack, providing a fresh surface that must becovered by an SEI layer. The growing SEI contains mass that does notcontribute to the energy stored in the battery and may present a barrierto lithium transport, thus reducing the rate performance of the battery.

Example Electrodes with Conversion Materials

FIG. 2A depicts four examples of electrode formats with conversionmaterials. Many variations, alternatives, and modifications arepossible. The particles or domains described above are nanostructured(e.g., separated from one another by less than about 20 nm lengthscale), and these particles or domains may be combined to form primaryand secondary particle structures shown in Examples 1-4 in FIG. 2A.While not depicted in these figures, the depicted particles may becoated with a protective material as described elsewhere herein.

Example 1 (top left of FIG. 2A) depicts an embodiment in which theelectrode active material includes non-encapsulated nanodomains oflithium fluoride, elemental metal, and metal fluoride. Such material mayexist at any state of charge, but will most typically exist at or nearfull discharge. Example 2 (top right) depicts an electrode format inwhich metal fluoride nanoparticles and lithium fluoride nanoparticlesare encapsulated in an elemental metal matrix. In each of theencapsulation examples, the encapsulation unit may exist as distinctparticles or as a continuous layer. Example 3 (bottom left) illustratesa format in which a metal fluoride matrix encapsulates lithium fluoridenanodomains and elemental metal nanodomains. Example 4 (bottom right)depicts a format in which lithium fluoride encapsulates metal fluorideparticles or nanodomains and elemental metal particles or nanodomains.

FIG. 2B depicts additional examples of particle and nanodomainstructures that may be employed in ferric fluoride and relatedconversion materials. In the example of FIG. 2B, the structure in theupper left side is a primary particle 211 that may be found in adischarged cathode. The primary particle 211 includes discretenanodomains of iron metal 213 and lithium fluoride 215. Often, a primaryparticle has a characteristic cross-sectional dimension of about 100 nmor less. As mentioned, the nanodomains that make up a primary particlehave cross-sectional dimensions of about 20 nm or less (e.g., about 5 nmor less). In some embodiments, a primary particle has a median diameterof between about 100 nm and about 1000 nm, or between about 200 nm andabout 300 nm. The median diameter is defined as the distance for whichabout 50% of the sample volume is contained within particles withdiameter smaller than that distance. In some cases, the nanodomains areglassy or compositionally homogeneous.

The top right structure in FIG. 2B depicts a secondary particle 217 (notdrawn to scale) of discharged ferric fluoride (FeF3) conversionmaterial. Secondary particles are made up of primary particles 211, suchas those presented in the top left structure, and possibly particles ofan ionically conductive material and/or electronically conductivematerial 219. Secondary particles may be agglomerates or clumps ofprimary particles and optionally particles of ionically/electronicallyconductive materials. In some implementations, secondary particles arepresent in a slurry used to coat a positive current collector whenforming the cathode. In certain embodiments, secondary particles have across-sectional dimension of about 0.1 micrometers to about 5micrometers. All dimensions presented in the discussion of FIG. 2B aremedian values. Any of the primary and/or secondary particles depicted inFIG. 2B may be coated with an isolating coating as described elsewhereherein.

The lower left and lower right structures presented in FIG. 2B representprimary particle 221 and a secondary particle 223, respectfully, offully charged ferric fluoride (FeF3) conversion material. Otherconversion materials may be substitute for ferric fluoride FeF3 and itsdischarge products in the structures presented in FIG. 2B.

Composition of Conversion Materials: Metal Component

In some embodiments, the “conversion reaction” may be written asaM+bLi_(c)X

M_(a)X_(b)+(b*c)Li⁺+(b*c)e ⁻

M_(a)X_(b)+(b*c)Li  (3)The left side of Reaction 3 represents the cathode active materials inthe discharged state, where the cathode active component includes ametal component M, and a lithium compound component Li_(n)X. In thisreaction, c is the formal oxidation state of anion X.

The right side of Reaction 3 represents the system in the charged state,where cathode active materials have been converted into the metalcompound component M_(a)X_(b) and the Li ions leave the cathode throughthe electrolyte, and electrons are provided to the external circuit.

X is generally any anionic species forming stable compounds Li_(n)X andM_(a)X_(b), with lithium and metal M respectively. Examples of suitableoxidizing species anions X in the conversion materials corresponding toReaction 1 include oxygen (O), sulfur (S), nitrogen (N), phosphorous(P), fluorine (F), selenium (Se), chlorine (Cl), iodine (I), andcombinations thereof.

Examples of suitable metal species M include transition metals,aluminum, and bismuth. In some cases, the metal is selected from thefirst row of transition metals. Specific examples of transition metalsthat may be used include bismuth (Bi), aluminum (Al), vanadium (V),chromium (Cr), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn),nickel (Ni), ruthenium (Ru), titanium (Ti), silver (Ag), molybdenum(Mo), and tungsten (W). In certain implementations, the metal componentis selected from iron (Fe), copper (Cu), manganese (Mn), and cobalt(Co). In a certain embodiments, the metal component is iron. In someembodiments, the metal component is copper. In some embodiments, themetal component is cobalt.

Alloys of such metals may also be used. Examples of such alloys includeiron alloyed with cobalt and iron alloyed with manganese. In certainembodiments, the metal component includes a mixture or alloy of a firstmetal and a second metal. In certain implementations of mixed metalcomponent, the metal component includes separate nanodomains of thefirst metal and the second metal. In another embodiment, the metalcomponent includes nanodomains of a mixture or alloy of the first andsecond metals. In certain embodiments, the first metal is Fe and thesecond metal is Cu.

Generally the lithium compound component is any lithium compound thatupon charging of the device yields (i) lithium ions that migrate to theanode, and (ii) anions that react with the metal component to provide ametal compound component. In the charged state, therefore, the cathodematerial includes a metal compound component. The anion in the lithiumcompound may generally be any anion that forms the lithium compound inthe discharged state and the metal compound in the charged state. Incertain implementations, the lithium compound is a lithium halide,lithium sulfide, lithium sulfur-halide, lithium oxide, lithium nitride,lithium phosphide, lithium hydride, lithium selenide, or mixturesthereof. In certain embodiments, the lithium compound is a lithiumhalide. In one version the lithium compound is lithium fluoride.

Specific examples of metal compounds M_(a)X_(b) that may be usedinclude, without limitation, the compounds listed in Table 1.

TABLE 1 Example Metal Compounds for Conversion Materials X═O X═S X═N X═PX═F Bi BiF₃ Ti TiF₃ V VF₃ Cr Cr₂O₃ CrS CrN CrF₃ Mn MnO₂, MnS MnP₄ Mn₂O₅,MnO Fe Fe₂O₃, FeS₂, Fe₃N FeP FeF₃, FeO FeS FeF₂ Co Co₃O₄, CoS₂, CoN,CoP₃ CoF₂, CoO Co_(0.92)S, Co₃N CoF₃ Co₉S₈ Ni NiO NiS₂, Ni₃N NiP₃, NiF₂NiS, NiP₂, Ni₃S₂ Ni₃P Cu CuO, CuS, CuP₂, CuF₂ Cu₂O Cu₂S Cu₃P Mo MoO₃,MoS₂ MoO₂ W WS₂ Ru RuO₂

Examples of suitable charged state cathode materials include sulfides,oxides, halides, phosphides, nitrides, chalcogenides, oxysulfides,oxyfluorides, sulfur-fluorides, and sulfur-oxyfluorides. In variousembodiments, the charged conversion material includes one or more of thefollowing: AgF; AlF₃; BiF₃; B₂O₃; CO₃O₄; CoO; CoS₂; CO_(0.92)S; Co₃S₄;Co₉S₈; CoN; Co₃N; CoP₃; CoF₂; CoF₃; Cr₂O₃; Cr₃O₄; CrS; CrN; CrF₃; CuO;Cu₂O; CuS; Cu₂S; CuP₂; Cu₃P; CuF₂; Fe₂O₃; FeO; FeOF; FeS₂; FeS; Fe₂S₂F₃;Fe₃N; FeP; FeF₂, FeF₃; Ga₂O₃; GeO₂; MnO₂; Mn₂O₃; Mn₂O₅; MnO; MnS; MnS₂;MnP₄; MnF₂, MnF₃, MnF₄, MoO₃; MoO₂; MoS₂; Nb₂O₅; NiO; NiS₂; NiS; Ni₃S₂;Ni₃N; NiP₃; NiP₂; Ni₃P; NiF₂; PbO; RuO₂; Sb₂O₃; SnF₂; SnO₂; SrO₂; TiS₂;TiF₃; V₂O₃; V₂O₅; VF₃; WS₂; ZnF₂; and combinations thereof. As anexample, a suitable cathode material is, in the charged state, ferricfluoride (FeF₃) in very small particles, which may be the size of aquantum dot (e.g., about 5 nm in the smallest cross-section) or in aglassy or amorphous state. In certain implementations, the metalcompound component is FeF_(x), where x is between 1 and 3. In certainembodiments, the metal compound component is CuF_(x), where x is between1 and 3. In certain implementations, the metal compound component isCoF_(x), where x is between 1 and 3.

The conversion material may be discharged with a cation that undergoesan exothermic reaction with the conversion material. The cation is oftenlow-cost and lightweight (relative small atomic mass). Lithium is notthe only example. Other examples include magnesium (Mg) and sodium (Na).

In certain embodiments, the three elements are intimately intermixed onan atomic scale. The relative amounts of the lithium compound componentand the metal component can vary widely, but should be appropriate for abattery cell. For example, the components may be provided in relativeamounts that do not introduce substantial unused material that will notcontribute to electrochemical energy conversion or enhance conductivity.In some embodiments where iron is used as the metal component, the moleratio of iron to lithium in the cathode active material is about 2 to 8,or about 3 to 8. In some embodiments employing valence 2 metals such ascopper, the mole ratio of metal to lithium in the cathode activematerial is about 1 to 5. In various implementations, the conversionmaterial is characterized by an iron (Fe) to fluorine (F) to lithium(Li) ratio of from about 1:1.5:1.5 to 1:4.5:4.5. As an example forferric fluoride (FeF₃) conversion material and Li cation, the conversionmaterial when created, or when in the discharged state, may be anamorphous mixture of lithium (Li), iron (Fe), and fluorine (F) in theratio of approximately 3:1:3 (Li₃FeF₃).

Composition of Conversion Materials: Lithium Metal Component

In some embodiments, at some point in the state of charge of theelectrode, the cathode includes an active component that includes alithium metal compound component. Generally, the lithium metal compoundcomponent is any compound that includes lithium, a non-lithium metal,and an anion. Upon charging the device, the lithium metal compoundcomponent yields lithium ions that migrate to the anode and a metalcompound.

In some embodiments, the reaction may be written asLi_(d)M_(e)X_(f)

dLi⁺ +de ⁻+M_(e)X_(f)

dLi+M_(e)X_(f)  (4)

The left side of Reaction 4 represents the cathode active materials inthe discharged state, where the cathode active component includes alithium metal component Li_(d)M_(e)X_(f) and the right side of Reaction4 represents the system in the charged state, where the cathode activematerials have been converted into the metal compound componentM_(e)X_(f), and the Li ions are provided for diffusion through theelectrolyte to the anode and the electrons are provided to the externalcircuit. In Reaction 4, all of the lithium in the lithium metal compoundis converted to lithium ions. In another embodiment, less than all ofthe lithium in the lithium metal component is converted to lithium ions.One version of such a reaction is given by Reaction 5:Li_(d)M_(e)X_(f)

gLi⁺ +ge ⁻+Li_(d-g)M_(e)X_(f)  (5)where g<d. Depending on the thermodynamic and kinetic stability of theLi_(d-g)M_(e)X_(f) compound, such compound may exist asLi_(d-g)M_(e)X_(f) or may be a mixture of one or more of a lithiumcompound, a metal compound, and a lithium metal compound.

In certain embodiments, the lithium metal compound component is alithium metal oxide, a lithium metal sulfide, a lithium metal nitride, alithium metal phosphide, a lithium metal halide or a lithium metalhydride, or mixtures thereof. In certain implementations, the lithiummetal compound component is a lithium metal halide. In certainembodiments, the lithium metal compound component is a lithium metalfluoride. In certain implementations, the lithium metal compoundcomponent is a lithium iron fluoride. In certain embodiments, thelithium metal compound component is a lithium copper fluoride. In oneversion the lithium metal compound component is a lithium cobaltfluoride. In one version the lithium metal compound component is alithium nickel fluoride.

Composition of Cathodes: Conducting Component

In some embodiments, the cathode includes a mixed electron-ionconducting component (“MEIC component”) together with an activecomponent as described above. The MEIC component may generally be madeof any material that is compatible with the other materials of thedevice and allows electron and lithium ion transport sufficient foroperation of the device. In certain implementations, the MEIC componentis a material having an electronic conductivity of 10⁻⁷ S/cm or greaterat the device operating temperature. In certain embodiments, the MEICcomponent is a material having a lithium ion conductivity of 10⁻⁷ S/cmor greater at the device operating temperature.

Examples of materials that may be used as the MEIC component include,without limitation, lithium titanates, lithium iron phosphates, vanadiumoxides, cobalt oxides, manganese oxides, lithium suphides, molybdenumsulphides, iron sulphides, LiPON, MoO₃, V₂O₅, carbon, copper oxides,lithium insertion compounds such as LiCoO₂, Li(CoMn)O₂, LiMn₂O₄,Li(CoNiMn)O₂, Li(NiCoAl)O₂, or other materials having relatively highlithium ion conductivity. In certain implementations, the MEIC componentis made of the same material as that of the solid state electrolyte. Incertain embodiments, the MEIC component is made of a different materialthan that of the solid state electrolyte. The MEIC component may itselfpossess electrochemical activity (e.g., MoO₃ or V₂O₅) or may not showelectrochemical activity (e.g., LiPON). In certain implementations, theMEIC is LiPON.

If the cathode includes an MEIC component, the minimum amount of MEICcomponent will generally be the amount that allows sufficient lithiumion and electron transport for functioning of the device. The maximumamount will be that amount of MEIC that provides an electrochemicallyactive cathode material with the required specific capacity or otherelectrical characteristics when operating at required rates, voltagewindows, and states of charge. In certain embodiments of the devicesincluding an MEIC, the minimum amount of MEIC is about 1% or about 5% ofthe cathode material by weight. In one version of the devices includingan MEIC, the maximum amount of MEIC is about 50% or 25% of the cathodematerial by weight.

The MEIC material may be provided in the electrode in various forms. Inone example, small particles of MEIC are mixed with theelectrochemically active particles and compressed. In another example,the MEIC arrays into vertical wires. The MEIC may include at least twomaterials, one having high electron conductivity and another having highionic conductivity.

In some embodiments of the device, the cathode includes an electronconductor dispersed to increase the electron conductivity of theelectrode. In some embodiments, the component has an electronconductivity value about 10⁻⁷ S/cm. This compound may be a carbon ormetal compound in some embodiments. Examples of forms of carbon that maybe employed include graphite, activated carbon, nanotubes, nanofibers,nanowires, graphene, graphene oxide, etc. A cathode may include activematerial with about 10% or about 20% of an electron conductor by weightor less. Examples of such material may be nanowires, nanoparticles, andnanocrystals and materials may be oriented in the direction from theelectrode to the electrolyte, or may be randomly dispersed. In certainembodiments, the material forms a percolating network throughout thecathode.

In some embodiments, the cathode includes a Li ionic conductor dispersedto increase the ion conductivity of the electrode. Example materials maybe nanowires, nanoparticles, or nanocrystals. These may be oriented inthe direction from the electrode to the electrolyte, or may be randomlydispersed. The ion material may be formed in coverings around activematerial particles. In certain embodiments, the material forms apercolating network throughout the cathode. In some embodiments, thematerial has an ion conductivity of at least 10⁻⁷ S/cm, or at least 10⁻⁵S/cm, or at least 10⁻⁴ S/cm at the operating temperature of the device.A non-limiting list of example materials includes lithium ironphosphate, carbon, Li₂O—SiO₂—ZrO₂, Li—Al—Ti—P—O—N, LiMO₂, Li₁₀GeP₂S₁₂,Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃, Li₇La₃Zr₂O₁₂, Li₉SiAlO₈, Li₃Nd₃Te₂O₁₂,Li₅La₃M₂O₁₂ (M=Nb, Ta), Li_(5+x)M_(x)La_(3-x)Ta₂O₁₂ (M=Ca, Sr, Ba),LiPON, lithium sulfide, lithium iron sulfide, iron sulfide, lithiumphosphate, Lisicon, thio-lisicon, glassy structures, lanthanum lithiumtitanate, garnet structures, β″ alumina, and lithium solid electrolytes.In some embodiments, the ion conductivity of the material is at leastgreater than the ion conductivity of the electrolyte. The ion conductormay be present in amounts of about 20% or less or about 10% or less ofthe active material in the cathode.

Anode

The anode may generally be made of any material that is compatible withthe other materials of the device and which may store lithium atoms orions when the device is in the charged state and may provide lithiumions for incorporation into the cathode when the device is in thedischarged state. In certain embodiments, the anode active material islithium metal. In another embodiment, the anode is lithium silicide,Li—Sn, or other high capacity, low voltage material that alloys withlithium. In an embodiment, the anode is lithium intercalated into acarbon component, such as graphite. In various embodiments, the anodeactive material is a material capable of inserting lithium ions at ahigher reversibility capacity than carbon, such as tin, magnesium,germanium, silicon, oxides of these materials, and the like.

In some embodiments, the anode is a porous material that allows lithiumplating into the pores, thereby relieving the swelling stress. Swellingstress may occur if the anode, such as lithium, plates on theelectrolyte, thereby causing swelling. In some embodiments, the poresare carbon nanotubes, carbon buckyballs, carbon fibers, activatedcarbon, graphite, porous silicon, aerogels, zeolites, or xerogels.

In certain implementations, the anode is formed in situ during the firstcharge cycle of the battery. In a case where the device is fabricated inthe discharged state with a lithiated cathode, the first charge cycleextracts lithium from the cathode and deposits lithium on the anodeside. In the case where the anode is a lithium metal anode, the anode isthereby formed in situ by plating on the anode current collector. Inthis case, the anode current collector may be a metal that does notalloy with or react with lithium. A non-limiting list of examples foranode current collector material includes TaN, TiN, Cu, Fe, stainlesssteel, steel, W, Ni, Mo, or alloys thereof. In certain embodiments,there is an excess of lithium in the device as fabricated on thecathode. In another embodiment, there is an excess of lithium in thedevice as fabricated on the anode side, possibly in the anode currentcollector. An excess of lithium may be desirable to prolong the cyclelife to the battery, as some lithium will inevitably be lost due to sidereactions, alloying with current collectors or in reactions with airand/or water that leak into the device. In certain embodiments, there isan encapsulation that substantially prevents ingress of air and waterinto the active materials. The encapsulation may be LiPON, an oxide,nitride, oxynitride, resin, epoxy, polymer, parylene, metal such as Tior Al, or multilayer combinations thereof.

Current Collectors

The devices described herein include optional positive and/or negativeelectrode current collectors. The current collectors generally may bemade of any material capable of delivering electrons to the anode or thecathode from the external circuit or delivering electrons to theexternal circuit from the anode and cathode. In certain implementations,the current collectors are made of a highly electronically conductivematerial such as a metal. In certain embodiments, the device does notinclude a cathode current collector and electrons are transferred to andfrom the cathode directly to the external circuit. In certainimplementations, the device does not include an anode current collectorand electrons are transferred to and from the anode directly to theexternal circuit. In certain embodiments, the device does not includeeither a cathode current collector or an anode current collector.

In certain implementations, the negative electrode current collector iscopper. In certain embodiments, the negative current collector is acopper alloy. In certain implementations, the negative current collectoris copper alloyed with a metal selected from nickel, zinc and aluminumor copper coated on a metal or polymer foil. In certain implementations,the current collector is copper and also includes a layer of anon-copper metal disposed between the copper and the cathode or anodematerial. In certain embodiments, the positive current collector iscopper and also includes a layer of nickel, zinc or aluminum disposedbetween the copper and the anode material.

In certain embodiments, the positive current collector is aluminum. Incertain implementations, the positive current collector is aluminum oran aluminum alloy. In certain embodiments, the positive currentcollector is aluminum and also includes a layer of a non-aluminum metaldisposed between the aluminum and the cathode or anode material. Incertain embodiments, the current collector is steel or stainless steel.In certain implementations, the current collector is steel or stainlesssteel and also includes a layer of a non-steel metal disposed betweenthe steel and the cathode or anode material. The cathode currentcollector and negative electrode current collector may be differentmaterials chosen among those enumerated above or any other suitablematerial.

Thin Film Cathodes

In some embodiments, a thin film of electrochemically active cathodematerial is formed between a positive current collector and theelectrolyte, which in turn contacts an anode or anode current collector.Optionally, an MEIC component may also be included. Any active componentand MEIC component described above may be used. The thin film may be acontinuous layer, which may be deposited by sputtering. Alternatively,it may be a layer including particles and/or nanodomains and isoptionally held together by a binder. In some embodiments, the thin filmcathode has a thickness of between about 2.5 nm and about 500 nm, orbetween about 5 nm and 300 nm, or about 200 nm or greater.

In certain embodiments, the cathode may contain a first thickness oflithium fluoride material configured in either amorphous orpolycrystalline state. The cathode may also include a first plurality ofnucleated iron metal species overlying the first thickness of lithiumfluoride material. The cathode may also have a second thickness oflithium fluoride material formed overlying the first plurality of ironmetal species. The second thickness of lithium fluoride material may beconfigured in either an amorphous or polycrystalline state to causeformation of a lithiated conversion material. The cathode region may becharacterized by an energy density of greater than 80% to about 100% ofa theoretical energy density of the cathode region. In some embodiments,the plurality of metal species nucleated overlying the first thicknessof lithium fluoride material causes formation of exposed regions of thefirst thickness of lithium fluoride material, or a thinner region ofiron metal species disposed between a pair of the plurality of ironmetal species. For example, each of the first thickness of lithiumfluoride material or the second thickness of lithium fluoride materialmay be between about 30 nm and 0.2 nm. Each of the first plurality ofiron metal species may have a diameter of about 5 nm to 0.2 nm. Theplurality of first iron metal species may be spatially disposed evenlyoverlying the thickness of the first thickness of lithium fluoridematerial. The entire thin film cathode may be coated with an isolatingcoating as described herein. Typically, such coating resides adjacent tothe electrolyte, e.g., electrolyte 102 in FIG. 1B.

Iron Migration

While charging, the metal component (e.g., iron) of the conversionmaterial may escape from the cathode and enter the electrolyte. In somecases, the metal may ultimately come into contact with the anode andcreate a short circuit between the anode and cathode and rendering thebattery inoperable. Iron may transport out of the cathode in variousforms such as a zero valence atom, iron(II) ion, iron(III) ion, and anycombination thereof.

FIG. 3A is a schematic illustration of the effect of iron migration.While the figure shows a particle of conversion material, which containsmany such particles, the concept applies to thin-films of conversionmaterial as well. In FIG. 3A, a section of a discharged cell isdepicted. In the discharged state, the particle includes metal atoms,such as iron, and metal halides, such as lithium fluoride. For purposesof this illustration, iron as a zero valence atom, an iron(II) ion, oran iron(III) ion are all labeled as “Fe” or “Metal.”

FIG. 3B schematically depicts the particle shown in FIG. 3A but whilecharging. When charging, lithium fluoride undergoes an electrochemicalreaction leaving fluoride ions in the particle, which subsequently bindto iron in the particle, while lithium ions and electrons leave theparticle, with the lithium ions entering the electrolyte and travelingto the anode. The electrons exit the cathode though a current collector.The depiction also shows some iron undesirably escaping the particlebefore it can bind to any fluoride ions in the particle left by thelithium fluoride. These iron atoms and/or ions may stay in theelectrolyte or transport to the anode, where they may accumulate tocause a short circuit. Either way, the exiting iron becomes unavailableto provide capacity. Therefore, it would be desirable to maintain ironatoms inside the particle or layer but allow lithium ions and electronsto flow freely in and out of the particle or layer.

Provided herein are coatings for electrochemically active conversionmaterial that limit or prevent iron egress from cathode particles orlayers and thereby improve efficiency of conversion materials in energystorage devices. Coatings prevent iron from leaving the cathodeparticles such that iron does not escape into the electrolyte. In someembodiments, the electrochemically active cathode material contains acollection of particles, each having a core including the conversionmaterial and the coating.

FIGS. 4A and 4B schematically illustrate the effect of using a coatingaccording to certain disclosed embodiments. FIG. 4A depicts a particleof conversion material in electrolyte. A coating may be deposited on theouter surface of a particle, and/or between the conversion material andthe electrolyte. In FIG. 4A, the coating is represented by the darkouter layer surrounding the particle itself. In some embodiments, thecoating may be deposited on a cathode layer, thereby forming a thinlayer between the cathode layer and electrolyte, such as in a thin-filmelectrode. In some embodiments, the cathode forms a substantiallycontinuous sheet, substantially coextensive with and overlapping theelectrolyte.

While charging, as depicted in FIG. 4B, lithium fluoride iselectrochemically reduced such that lithium ions leave the particles,electrons leave the particles, and fluorine ions react with iron to formcompounds such as ferric fluoride (FeF₃). As shown, iron escape is atleast partially prevented by the coating, but the coating is selectivelypermeable to both lithium ions and electrons to promote conductivity andallow for the effective use of the cell as an energy storage device. Ascompared to FIG. 3B, iron atoms or ions that were able to escape from aparticle or conversion material without a coating are now prevented fromescaping the conversion material due to the additional coating.Generally, “selectively isolate” means isolating a component of aparticle from some species but not others. As shown in FIG. 4B, thecoating selectively isolates a conversion material by at least partiallypreventing iron from entering the electrolyte, while permitting lithiumions and electrons to enter and leave.

Electrolyte Reaction

Conversion materials may also be susceptible to a reaction with theelectrolyte on the surface of particles or thin-films. Reaction 6provides an example of a reaction that may occur between the cathode anda sulfur-containing electrolyte:MF_(x)+S→M_(1-y)S_(1-y)+F_(x-y)+S_(y)  (6)where M is a metal such as any of those described above. As an example,Reaction 7 provides an example of a reaction that may occur between acathode including ferric fluoride and an electrolyte:FeF₂+2S→FeS₂+F₂  (7)

FIGS. 5A and 5B are example schematic illustrations of this phenomenon.In FIG. 5A, a particle of conversion material including metal atoms isshown in electrolyte with various electrolyte molecules surrounding theparticle. This state may exist soon after the fabricated cathode isbrought in contact with the electrolyte. Next, in FIG. 5B, after thecathode has remained in contact with the electrolyte for some time,electrolyte molecules at the surface of the particle may enter theparticle and/or interact with the particle surface. Either way, theyundergo reaction with metal at the surface of the particles andconversion material, thereby rendering the particle and conversionmaterial less effective as an energy storage unit.

Provided herein are coatings for cathode conversion materials that limitor prevent electrolyte interaction with conversion materials and therebyimprove efficiency of conversion materials in energy storage devices.Coatings isolate conversion material from electrolyte in particlesand/or thin-film layers, and may provide various benefits such asincreased cycle lifetime and increased efficiency.

FIGS. 6A and 6B are schematic illustrations of the effect of usingcoatings to prevent electrolyte reactions. In FIG. 6A, a particlecontaining conversion material or metals is shown with electrolytemolecules located outside of the particle. A coating may be deposited onthe outer surface of a particle, or between the conversion material andthe electrolyte. In FIG. 6A, the coating is represented by the darkouter layer surrounding the particle itself. In some embodiments, thecoating may be deposited on the cathode, thereby forming a thin layerbetween the cathode layer and electrolyte, such as in a thin-filmelectrode. FIG. 6B depicts the effective use of the coating. Arrows showthe paths that some of the electrolyte molecules may exhibit toward theparticle, where they would react with metal on the surface of theparticle, but, due to the coating, are unable to react with the surfaceof the particle, and therefore unable to undergo a reaction thatdecreases efficiency. The coating selectively isolates the components inthe particle.

While the discussion in this section focuses on reaction of conversionmaterial with electrolyte, it applies equally to reactions of conversionmaterial with other cell components such as cathode binders, MEICmaterials, electron conductor additives, and ion conductor additives.Also, the electrolyte that interacts with the conversion material may befashioned as a catholyte or a single electrolyte for the entire cell.

Fusion of Active Material

In a particle format, two or more particles of conversion material(e.g., primary materials) may fuse to form a larger particle when a highcurrent is run to activate the active material. In some embodiments, thefusion of particles may result in larger conversion material particles,which decreases the conductivity of the electrode because ions andelectrons travel a longer distance when larger particles are used in thecathode.

Provided herein are coatings for cathode conversion materials thatprevent particle fusion and improve ionic and electronic conductivity.The coatings are not active material and therefore coated particles thatare close to each other while a high current is run will not be fusedtogether and particles remain their small sizes. Thus, coatings preventprimary particles from fusing during charging and discharging.

Coating Materials

The coating material may include one or more of the following classes ofchemicals: oxides, phosphates, and fluorides. Specific examples ofmaterials that may be used as coatings include aluminum oxide(Al_(x)O_(y) or Al₂O_(y), y≤3 (e.g., Al₂O₃)), lithium oxide (Li_(x)O,x≤2), titanium oxide (TiO_(x), x≤2), silicon oxide (SiO_(x), x≤2), ironoxide (Fe₂O, x≤3), aluminum phosphate (AlPO_(x), x≤4; or Al_(x)(PO₄)_(y)(e.g., AlPO₄)), iron phosphate (FePO_(x), x≤4), lithium fluoride(Li_(x)F, 0.8≤x≤1.2), aluminum fluoride (AlF_(x), x≤3 (e.g., AlF₃)), andcarbon. For example, the coating may be aluminum oxide (Al₂O₃), oraluminum phosphate (AlPO₄), or aluminum fluoride (AlF₃).

During fabrication or initial cycling, lithium ions embed in thecoating, and thereafter the coatings maintain good cycle capacity. Aninitial lithiating operation (e.g., an initial cycle) may be employed toform a pre-lithiated coating material. Examples of lithiated coatingmaterials include lithiated aluminum oxide (Al₂O₃), lithiated aluminumphosphate (AlPO₄), and lithiated aluminum fluoride (AlF₃), or mixturesthereof. A lithiated coating may be defined as a coating includinglithium, which may be intercalated, alloyed, mixed, or otherwiseincorporated in the coating. Examples of lithiated aluminum oxidecoatings include lithium reductively intercalated into aluminum oxide(e.g., LiAl₂O₃), lithium oxide alloyed with aluminum oxide (e.g.,Li₃Al_((2-x))O₃), lithium aluminum oxide compounds (e.g., LiAlO₂,Li₄Al₂O₅), and lithium oxide mixed with aluminum oxide (e.g.,Li₂O.Al₂O₃). In certain embodiments, x is between about 1 and 6. Similarlithium to metal ratios may be employed in other coating compositions.Use of pre-lithiated coatings increases the coatings' lithium ionconductivity and allows use of the full capacity of the conversionmaterial without requiring lithiation during each cycle in normaloperation. An example of a pre-lithiation level may be about 50%, whichis the level of lithiation where lithium ion conductivity is thehighest. Lithiated coatings may be fabricated to have sufficient lithiumavailable to immediately conduct, but not so much lithium that asubstantial amount of conduction sites in the coating are filled. Thus,a 50% lithiated coating results in high conductivity because sites areformed including lithium, but are only half-filled.

Coating Properties

The layer of coating material should be as thin as possible withoutsacrificing its function to protect the conversion materials or cathode.A thin coating should be used because the coating material typicallydoes not provide electrochemical energy capacity, and effectivelypresents dead weight/volume in the cell if too thick. In manyembodiments, the coating material layer has a median thickness betweenabout 0.5 nm and about 15 nm, depending on the size of the cathode orcathode particles. In some embodiments, the coating material layermedian thickness is between about 1 nm and about 10 nm, or between about2 nm and about 7 nm.

Coating material used in disclosed embodiments may also have a porositybetween about 0% and about 30%. The porosity may be chosen to providegood lithium ion permeability but poor metal atom or metal ion andelectrolyte permeability. The coating may have a diffusion coefficientfor lithium ions between about 10⁻¹⁰ and about 10⁻⁵ cm²/s. In someembodiments, the coating has a diffusion coefficient for iron(II) ionsbetween about 10⁻¹⁴ and about 10⁻¹⁰ cm²/s. Conversion material havingcoatings may have a lower active material loss rate than conversionmaterial without such coatings. The chemical structure of the coatingmaterial may also be crystalline, semicrystalline, and/or amorphous.

The ionic conductivity of the coating material with respect to iron(II)ions may be no greater than about 10⁻¹⁰ S/cm or no greater than about10⁻⁸ S/cm. Coated conversion materials may be relatively ionicallyconductive to lithium ions. Resulting ionic conductivity of the coatingmaterial with respect to lithium ions may be at least about 10⁻⁸ S/cm orat least about 10⁻⁷ S/cm. In some embodiments, the coated material mayhave a sufficient Li ion conductivity such that the battery capacitywhen discharged at 1 C rate is at least 60% of the capacity as whendischarged at a C/3 rate. Coated conversion materials may also berelatively electronically conductive. The electronic conductivity of thecoating material may be at least about 10⁻⁸ S/cm or at least about 10⁻⁷S/cm. In certain embodiments, the active material of the coatedconversion materials has an energy density of at least about 800 Wh/kg,or at least about 1000 Wh/kg. In a coating with two or more layers, eachlayer may have a different ionic and electronic conductivity. Forexample, a bilayer coating may have a high-conductivity inner layer anda lower-conductivity outer layer.

In many embodiments, coatings may be continuous over the entire surfaceof the particle or entire surface of the cathode material. In someembodiments, coatings may be discontinuous on the surface of theparticle or cathode material. In many embodiments, coatings may have amedian coverage of at least about 80% of the surface area, or at leastabout 90% of the surface area.

The coating may include one or more layers such as a bilayer. In someembodiments, the coating may include two or more layers. In a bilayercoating, one of the layers may block iron or copper diffusion. In someembodiments, one of the layers may be a good electronic conductor. In anembodiment where one layer blocks iron diffusion and another layerprovides good electronic conductivity, electrons may flow around theperimeter of the cathode layer or particle in a manner that allows theiregress into the interior of the particle at various points of entry onthe particle surface. In a coating including one or more layers, eachlayer of the coating may have a median thickness between about 0.5 nmand about 15 nm, depending on the size of the cathode or cathodeparticles. In some embodiments, the median thickness of each layer isbetween about 1 nm and about 10 nm, or between about 2 nm and about 7nm.

FABRICATION METHODS

Various classes of methods of forming coatings may be used. Coatingmaterial may be prepared on charged or discharged conversion material,or in an unlithiated or lithiated conversion material. Further, thecoating may be applied in a lithiated, partially lithiated, orunlithiated form. In many embodiments, the coating is deposited at a lowtemperature, for example less than about 300° C., or less than about200° C., to prevent agglomeration of components within a dischargedcathode conversion material.

The coatings may be formed on conversion materials as fabricated, suchas in-situ as the active material is being formed, or coatings may beapplied to previously fabricated conversion material particles, in whichcase conversion material particles having above-described properties areinitially formed.

Coatings may be deposited or fabricated using processes such as spraycoating, sol-gel, colloidal dispersion, precipitation reaction, physicalor chemical adsorption, mechanical alloying or milling, physicaldeposition, physical vapor deposition (PVD), chemical vapor deposition(CVD), atomic layer deposition (ALD), and the like. Processes such asPVD, CVD, and ALD may be performed with or without the assistance of aplasma. When coating particles, a fluidized bed process may be employed,particularly with processes such CVD and ALD.

Spray Coating

To perform spray coating, a solution of coating salts is prepared in anappropriate solvent (water, methanol, ethanol, or isopropyl alcohol). Insome embodiments, the solution includes lithiated iron fluoride, whichmay have a lithium to iron atomic ratio of between about 1:1 and about3:1. Aluminum oxide (Al₂O₃) may be formed from aluminum nitrate(Al(NO₃)₃. 9 H₂O) directly. A phosphate film (AlPO₄) may be formed bymixing aluminum nitrate and phosphoric acid (H₃PO₄). A fluoride film(AlF₃) may be formed from aluminum nitrate and ammonium fluoride (NH₄F)or ammonium hydrogen difluoride (NH₄HF₂). Lithiated forms of the filmsmay be formed by including the appropriate amount of lithium nitrate(LiNO₃). The solution is then spray-deposited using an atomizer spraynozzle or nebulizer onto a heated substrate above the boiling point ofthe solvent.

To perform spray coating, a solution of coating precursors is preparedin an appropriate solvent (water, methanol, ethanol, or isopropylalcohol). Aluminum oxide (Al₂O₃) may be formed from aluminum nitrate(Al(NO₃)₃.9H₂O) directly. The phosphate film (AlPO₄) maybe formed bymixing aluminum nitrate and phosphoric acid (H₃PO₄). A fluoride film(AlF₃) maybe formed from aluminum nitrate and ammonium fluoride (NH₄F)or ammonium hydrogen difluoride (NH₄HF₂). The lithiated forms of thefilms maybe formed by including the appropriate amount of lithiumnitrate (LiNO₃) or other suitable lithium compound. The solution is thenmixed with the active material and spray-dried through a heated atomizerand the particles are collected in a cyclone or an electrostatic device.

Precipitation

To perform a precipitation reaction, solutions are prepared with analuminum compound dissolved in a suitable solvent such as toluene. Thealuminum compound can be a halide such as AlCl₃, AlBr₃, or AlI₃.Optionally, a lithium compound can be included to produce a lithiatedcoating. In some embodiments, the lithium compound is a lithiated ironfluoride with a lithium to iron atomic ratio of between about 1:1 andabout 3:1. Separately, a second coating component is dissolved in asuitable solvent. In one example for fabricating an aluminum phosphatecoating, a solution of 85 wt % phosphoric acid in water is mixed indimethylcarbonate. The active material to be coated is then suspended ina suitable solvent such as toluene. The active material may be preparedusing a mixture of lithiated iron fluoride with 6-aminohexanoic acid orbenzylamine in a solvent such as toluene. The precursor solutions arethen added to the solution, optionally via a metered method such as asyringe pump or addition funnel. Many other precursors may be suitablefor fabricating coated cathodes in accordance with the disclosedembodiments. For example, a coating of aluminum oxide can be formed bythe liquid-phase reaction of trimethylaluminum (TMA) and water.Similarly, the lithium compound can be an organometallic precursor suchas butyllithium.

Precursors for a precipitation reaction can also be formed in situ byreaction or decomposition of a starting material. For example, analuminum alkoxide such as aluminum methoxide or aluminum isopropoxidecan be used as the source for an aluminum oxide synthesis, optionallywith heat and/or added water assisting in the decomposition. In anotherexample, aluminum acetylacetonate is reacted with an amine such asbenzylamine to form aluminum oxide.

Adsorption

To perform an adsorption reaction, the active material is dispersed in asuitable solvent such as toluene or an alcohol, and then contacted withnanoparticles of a suitable coating material under conditions whichpermit the coating particles to adsorb to the active material surface.The coating particles may be between about 0.5 and about 500 nm, orbetween about 0.5 and about 50 nm, or between about 0.5 and about 5 nm.In some embodiments, the slurry may be heated prior to contacting thenanoparticles of coating material. In some embodiments, the slurry iscontacted with nanoparticles under conditions leading to formation ofparticles free in solution, which adsorb the cathode particle. In someembodiments, the reaction takes place primarily at the particle surfaceso as to form only as a coating material. In some embodiments, thecoating particles can first be formed in a separate reactor using eitherdry powder, which resuspends in the solvent containing the activematerial, or a pre-formed suspension in a suitable solvent and thensubsequently added to the active material. Alternatively, the coatingparticles can be formed in situ by a reaction of suitable precursors.For example, an aluminum alkoxide such as aluminum methoxide or aluminumisopropoxide can be used as the source for an aluminum oxide synthesis,optionally with heat and/or added water assisting in the decomposition.In another example, aluminum acetylacetonate is reacted with an aminesuch as benzylamine to form aluminum oxide.

Atomic Layer Deposition (ALD)

To perform ALD, the active material to be coated may be suspended in afluidized bed reactor and heated to a suitable temperature. In someembodiments, the active material may be a lithiated iron fluoride with alithium to iron atomic ratio of between about 1:1 and about 3:1. Oneexample forms an aluminum oxide coating via the reaction of TMA withwater at about 150° C. to about 200° C., for example 180° C. In thisprocess, TMA is fed into the reactor to coat the particle surfaces,optionally using a residual gas analyzer (RGA) at the outlet of thereactor to detect when TMA is able to reach the exit of the reactor,which indicates complete surface reactivity. The reactant is thenchanged to H₂O and the process gas is flowed until water saturates theparticle surfaces, again optionally detected by an RGA. The tworeactants are then alternated until the desired thickness or weightcoating is obtained.

Other Fabrication Techniques

To create a coating via PVD, a substrate to be coated is placed in asputter chamber. A target of aluminum is then sputtered in an oxidizingenvironment, such as an oxygen plasma. Aluminum oxide (Al₂O₃) will thenbe deposited on the substrate.

When the conversion material is formed first prior to application of thecoating, various processes may be suitable for fabricating suchmaterial. Some fabrication techniques employ exclusively materialsynthesis. Other processes employ exclusively coating of the conversionmaterial on a substrate. Examples of material synthesis processesinclude sol gel synthesis, one pot synthesis, bottom-up synthesis, andmelt spinning Examples of substrate coating process include slot-die,spin coating, dip coating, doctor blade, metering rod, slot casting,screen printing, inkjet printing, aerosol jet, knife-over roll, commacoating, reverse comma coating, tape casting, slip casting, gravurecoating, and microgravure coating. Various processes employ a hybrid ofsynthesis/coating. Suitable processes for particle formation/downsizinginclude dry milling, wet milling, high energy milling, or bottom-upchemical synthesis.

In certain embodiments, a coated conversion material for a cathode isprepared using a process in which one or more precursors or reactantsare contacted in solid phase, also referred to as “solid phasesynthesis.” Examples include hot pressing, cold pressing, isostaticpressing, sintering, calcining, spark plasma sintering, flame pyrolysis,combustion synthesis, plasma synthesis, atomization, and melt spinningSome solid phase syntheses involve grinding and mixing of bulk precursormaterials. The bulk materials are ground to very small dimensions andthen combined or otherwise mixed and reacted as necessary to form thedesired composition. Milling may be performed via jet milling,cryomilling, planetary milling (Netzsch, Fritsch), high energy milling(Spex), and other milling techniques. In some embodiments, the groundand mixed particles are calcined. An examples of solid phase synthesisprocesses for producing iron fluoride conversion materials are set forthin U.S. Provisional Patent Application No. 61/814,821, filed Apr. 23,2013, and titled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICALCONVERSION REACTIONS,” and U.S. Provisional Patent Application No.61/803,802, filed Mar. 21, 2013, and titled “METHOD FOR FORMING IRONFLUORIDE MATERIAL,” both of which are incorporated herein by referencein its entirety.

In certain embodiments, the conversion material is produced using adeposition technique such as evaporation, sputtering, Chemical BathDeposition (CBD), or vapor phase deposition technique such as PVD, ALD,or CVD. In one method described herein, the devices are fabricated bysputtering using an Endura 5500 by Applied Materials of San Jose, Calif.

In certain embodiments, the devices are fabricated by sequentialdeposition of the anode current collector, anode (optional),electrolyte, cathode, coating, and cathode current collector on asubstrate. In certain implementations, there is no separate substrateand the anode, electrolyte, cathode, coating, and cathode currentcollector are deposited directly on the anode current collector. In oneversion, there is no separate substrate and the cathode, coating,electrolyte, anode, and anode current collector are deposited directlyon the cathode current collector.

The following is a list of examples of suitable conversion materialfabrication methods categorized by the process environment:

-   -   Vacuum processes, including sputtering, evaporation, reactive        evaporation, vapor phase deposition, chemical vapor deposition        (CVD), plasma-enhanced CVD (PECVD), metal organic CVD (MOCVD),        atomic layer deposition (ALD), plasma-enhanced ALD (PEALD),        molecular beam epitaxy (MBE), ion-beam-assisted deposition        (IBAD), and pulsed laser deposition (PLD).    -   Wet synthesis, including chemical bath deposition (CBD),        electroplating, spraying and in situ formation, Langmuir,        Langmuir-Blodgett, layer-by-layer, electrostatic spray        deposition, ultrasonic spray deposition, aerosol spray        pyrolysis, sol gel synthesis, one pot synthesis, and other        bottom-up methods.    -   Dry synthesis, including pressing, hot pressing, cold pressing,        isostatic pressing, sintering, spark plasma sintering, flame        pyrolysis, combustion synthesis, plasma synthesis, atomization,        and spin-melting.    -   Top-down methods, such as jet milling, wet/dry milling,        planetary milling, and high energy milling.    -   Other methods, such as precipitation, colloidal dispersion,        physical or chemical adsorption, and mechanical alloying.        Applications

The devices described herein may generally be used in any applicationrequiring energy storage. The devices may be particularly well suitedfor in applications such as in electric vehicles, hybrid electricvehicles, consumer electronics, medical electronics, and grid storageand regulation.

EXPERIMENTAL

Energy Retention and Energy Density

An experiment was conducted to evaluate the energy retention and energydensity over cycles for both uncoated and coated conversion materialsaccording to disclosed embodiments. An uncoated cathode including ironand lithium fluoride was fabricated for an energy storage device. Tocompare, a 2 nm and 4 nm layer of FeF₃ was deposited as a coating oncathode particles of another energy storage device including iron andlithium fluoride using atomic layer deposition (ALD). The energy densityand energy retention were measured over cycles of charge and discharge.FIG. 7A depicts the nominally poor cycle life of the energy storagedevice at high temperature without a protective coating layer on thecathode particles. By contrast, FIG. 7B depicts a dramatically improvedcycle life of the energy storage device at high temperature with theprotective coating layer on cathode particles. Note significantdifference in energy retention beyond 400 cycles of charge anddischarge. No substantial decrease in energy retention was observed forthe coated conversion materials.

Spray Coating

In one example, 5 g of lithiated iron fluoride was weighed out and addedto 20 g of isopropanol. The lithiated iron fluoride had a lithium toiron atomic ratio of 1:1. The solution was vigorously mixed to produce aslurry. Then 11 mL of an 0.5 M solution of Al(NO₃)₃.9H₂O in isopropanolwas added to the slurry. This mixture was used as-is to prepare an Al₂O₃coating in one example. In another example, there was a further additionof 7.4 mL of a 1.0 M solution of H₃PO₄ in isopropanol to prepare amixture to form an AlPO₄ coating. In yet another example, there was afurther addition of 5.5 mL of 1.0 M LiNO₃ to prepare a mixture to form alithiated AlPO₄ coating with a lithium to aluminum atomic ratio of 1:1.In each case, the mixture was then sprayed through a 0.7 mm atomizingnozzle on an inlet heated to 200° C., collected in a cyclone, dried, andcollected.

In another example, a solution was prepared by mixing 113 mg of H₃PO₄,52 mg LiNO₃, and 188 mg of Al(NO₃)₃.9H₂O in 10 mL of ethanol. Then 420 Lof the solution was mixed with 45.80 ml, of additional ethanol. A 4″×4″substrate containing lithiated iron fluoride having a 3:1 lithium toiron atomic ratio to be coated was held onto a heated stage using avacuum chuck. The stage was heated to 225° C. Using a Burgener Mira Mistnebulizer (from Burgener Research Inc. of Mississauga, Ontario, Canada),31.1 grams of the spray solution was deposited by rastering thesubstrate in the ±X direction while rastering the nebulizer in the ±Ydirection.

Precipitation

In one example, 5 g of lithiated iron fluoride was weighed out and addedto 50 mL of toluene under an argon atmosphere. The lithiated ironfluoride had a lithium to iron atomic ratio of 3:1. The solution wasvigorously mixed to produce a slurry. One syringe was loaded with 2.75mL of a 2.0 M solution of AlBr₃ and a second syringe was loaded with27.5 mL of a 0.2 M solution of H₃PO₄ in dimethylcarbonate. Thesemixtures were used to prepare an aluminum phosphate coating in oneexample. In another example, a lithiated aluminum phosphate coating wasprepared by loading one syringe with 2.75 mL of a solution that included2.0 M AlBr₃ and 1.0 M LiBr and loading the second syringe with 27.5 mLof a 0.2 M solution of H₃PO₄ in dimethylcarbonate. The lithiatedaluminum phosphate coating was prepared to a lithium to aluminum atomicratio of 1:2. In each case, syringe pumps were used to add bothsolutions dropwise over the course of 60 minutes, and then the productwas centrifuged, washed with toluene, dried, and collected.

In one example, 5 g of lithiated iron fluoride was weighed out and addedto 50 mL of toluene under an argon atmosphere. The lithiated ironfluoride had a lithium to iron atomic ratio of 3:1. The solution washeated to 80° C. and stirred vigorously to produce a slurry. One syringewas loaded with a solution of 5.4 g of aluminum acetylacetonate in 40 mLtoluene and a second syringe was loaded with a solution of 6 g ofbenzylamine in 15 mL toluene. Syringe pumps were used to add bothsolutions dropwise over the course of 60 minutes, and then the solutionwas centrifuged, rinsed with toluene, dried, and collected.

In one example, 5 g of lithiated iron fluoride was weighed out and addedto 25 mL of benzylamine under an argon atmosphere. The lithiated ironfluoride had a lithium to iron atomic ratio of 3:1. The solution washeated to 130° C. and stirred vigorously to produce a slurry. A syringewas loaded with a solution of 5.4 g of aluminum acetylacetonate in 40 mLbenzylamine. A syringe pump was used to add the solution dropwise overthe course of 60 minutes, and then the solution was centrifuged, rinsedwith toluene, dried, and collected.

In one example, 0.8 g of lithiated iron fluoride was weighed out andadded to 20 mL of benzylamine under an argon atmosphere. The lithiatediron fluoride had a lithium to iron atomic ratio of 3:1. The solutionwas heated to 130° C. and stirred vigorously to produce a slurry. Asyringe was loaded with a solution of 0.43 g of aluminum acetylacetonatein 5 mL benzylamine. A syringe pump was used to add the solutiondropwise over the course of 15 minutes. The solution was held at 130° C.for an additional 120 minutes, then centrifuged, rinsed with toluene,dried, and collected.

In one example, 1.0 g of lithiated iron fluoride was weighed out andadded to a solution of 0.032 g of 6-aminohexanoic acid in 25 mL oftoluene under an argon atmosphere. The lithiated iron fluoride had alithium to iron atomic ratio of 3:1. The solution was heated to 80° C.and stirred vigorously to produce a slurry. A syringe was loaded with asolution of 0.22 g benzylamine, 0.53 g aluminum acetylacetonate, and 10mL of toluene. A syringe pump was used to add the solution dropwise overthe course of 60 minutes. The solution was held at 80° C. for anadditional 60 minutes, then centrifuged, rinsed with toluene, dried, andcollected.

Atomic Layer Deposition

In one example, 50 g of lithiated iron fluoride was weighed out andadded to a fluidized bed ALD reactor from ALD Nanosolutions ofBroomfield, Colo. under an argon atmosphere. In one example, thelithiated iron fluoride had a lithium to iron atomic ratio of 3:1. Inanother example, the lithiated iron fluoride had a lithium to ironatomic ratio of 1:1. The gas flow was adjusted to ensure adequatesuspension of the particles with minimal loss of fines. The reactor washeated to 200° C. TMA was introduced into the chamber by feeding intothe circulating argon flow. A RGA was used to observe the byproducts ofthe surface reaction, and the TMA feed was stopped when TMA was observedby the RGA. H₂O was introduced into the chamber by feeding H₂O vaporinto the circulating argon flow. Again, a RGA was used to observe thebyproducts of the surface reaction, and the H₂O feed was stopped whenH₂O was observed by the RGA. The process was repeated for a number ofsteps to achieve the desired coating thickness. In one example, theprocess was repeated for 10 cycles to deposit a coating having athickness of approximately 1 nm.

Physical Vapor Deposition

In one example, an aluminum source was prepared by bonding a high purity(>99.9%) aluminum target (3 inch diameter disc with a thickness of 0.25inch or 0.125 inch) onto a backing plate. The target was then mountedinto a sputtering chamber using the Endura 5500 by Applied Materials ofSan Jose, Calif. The substrate, which contained lithiated iron fluoridehaving a lithium to iron atomic ratio of 3:1, was then introduced intothe chamber and a vacuum was pulled on the chamber with a base pressureless than 10⁻⁷ Torr. An Ar/O₂ plasma was then ignited using a magnetron.The plasma then reactively sputtered the aluminum source to depositAl₂O₃ onto the substrate. The thicknesses were determined by the lengthof the plasma pulse. Times were on the order of less than a few minutesfor a thickness of 10 nm or less.

CONCLUSION

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, it will be apparent thatcertain changes and modifications may be practiced within the scope ofthe appended claims. It should be noted that there are many alternativeways of implementing the processes, systems, and apparatus of thepresent embodiments. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the embodiments arenot to be limited to the details given herein.

What is claimed is:
 1. An energy storage device comprising: a. an anode,b. an electrolyte, and c. a cathode comprising: a plurality of coatedelectrochemically active material particles, each electrochemicallyactive material particle comprising: a core comprising a conversionmaterial, and a coating surrounding the core comprising the conversionmaterial, wherein the coating selectively isolates the conversionmaterial from the electrolyte, wherein the electrochemically activematerial particles have a capacity that is greater than 300 mAh/g;wherein the coating has a median coating coverage surrounding the corecomprising the conversion material that is at least 90% of the surfacearea of the core comprising the conversion material; and wherein thecore comprising the conversion material comprises conversion material inthe charged state; wherein the conversion material comprises a fluorideof a metal; and wherein the metal is iron, manganese, nickel, copper, orcobalt; wherein the coating surrounding the core comprises lithiumoxides, lithium halides, lithium alloys, or combinations thereof.
 2. Theenergy storage device of claim 1, wherein the device has a capacity whendischarged at 1 C that is at least 60% of its capacity when dischargedat C/3.
 3. The energy storage device of claim 1, wherein the coatingsurrounding the core comprises two or more layers, each layer having athickness between about 0.5 nm and about 15 nm.
 4. The energy storagedevice of claim 1, wherein the cathode further comprises an ionconductor, and a current collector.
 5. The energy storage device ofclaim 4, wherein the cathode forms a continuous sheet overlapping theelectrolyte.
 6. The energy storage device of claim 1, wherein theconversion material comprises nanocrystalline domains having a mediansize of <20 nm.
 7. The energy storage device of claim 6, wherein theconversion material is a mixture of two metal fluorides.
 8. The energystorage device of claim 4, wherein the conversion material comprisesnanocrystalline domains having a median size of <20 nm.
 9. The energystorage device of claim 1, wherein the core comprising conversionmaterial comprises FeF₃.
 10. The energy storage device of claim 1,wherein the coating completely surrounds the core comprising theconversion material.